1. Field of the Invention
The present invention relates to an information storage medium such as a multi-layer optical disc including Compact Disc, Compact Disc-Recordable, DVD, DVD-Recordable, DVD-RAM, etc., on and from which information is recorded and reproduced. More specifically, the present invention relates to the focus servo control technique for focusing the light beam on the respective layers of the multi-layer optical disc (so-called "focus search").
2. Description of the Prior Art
Recently, there are known a multi-layer information storage medium having multiple recording layers and an information recording/reproducing apparatus for recording and/or reproducing information on and/or from the multi-layer storage medium. Such information recording/reproducing apparatus is provided with an optical pickup generally including a laser diode for emitting a light beam, a polarization beam splitter, an objective lens and photodetectors. At the time of reproducing information, the optical pickup emits the light beam and receives the reflected light from the optical disc to reproduce recorded information. At the time of recording information, the apparatus modulates the light beam by the information to be recorded, and then irradiates the light beam thus modulated on the storage medium to record information.
When irradiating the light beam on the storage medium, it is necessary to focus the light beam on the recording surface of the optical disc (so-called "focus search") and then constantly maintain the focused state of the light beam on the layer of the storage medium (so-called "focus servo control"). The conventional information reproduction apparatus generally moves the objective lens vertically in relation to the layer of the optical disc to execute the focus search and the focus servo control. The focus search is also required to restore the focused state of the light beam and restart the focus servo control when the focused state is lost (i.e., out-of-focus state).
In recording and/or reproducing information on and/or from the multi-layer optical disc, the focus search should be performed to detect a certain target layer with which the recording and/or the reproduction is carried out. In such case, the conventional apparatus first moves the light beam, and accordingly moves the focal point of the light beam, toward the position of the pickup beyond the layer of the disc closest to the optical pickup, and subsequently starts the focus search from one side of the disc near to the pickup to the other side of the disc remote from the pickup. Specifically, the focal point is temporarily moved toward the pickup beyond the layer of the disc closest to the pickup so that the focal point is positioned out of the optical disc. Subsequently, the objective lens is moved remotely from the pickup so that the focal point passes through the respective layers of the disc and the so-called "S-curves" in the focus error signal is detected and counted, thereby permitting the focal point to be positioned on the target layer.
In this method, however, the detection of the target layer may sometimes take relatively long period of time. Namely, assuming that the focused condition is lost during the reproduction of the fifth layer of the 6-layered disc, the pickup is temporarily moved to the original position and then moved toward the disc until the fifth S-curve, corresponding to the fifth layer, is detected. Namely, the focal point of the light beam is once moved out of the disc and then moved to traverse four layers before reaching the fifth layer. In this way, the movement of the pickup is relatively large, particularly for the disc having large number of layers, and the time required to restore the focused state is necessarily relatively long.